1. Field of the Invention
This invention relates generally to the field of image analysis methods and more particularly to methods of creating a mask surrounding an area of interest in an image, such that when the mask is applied to the image (e.g., though a summation operation) only the area of interest remains. The invention is applicable to digital images generally. One area where it is of particular utility is analysis of images of biological specimens. For example, the mask can serve to isolate and highlight areas of interest, such as cancerous cells, in a magnified image of a cellular or tissue specimen.
2. Description of Related Art
In the biology fields, including cytology, histology, and pathology, digital images of tissue and cellular specimens are typically obtained from a microscope equipped with a color camera which records red, green, and blue planes for these images. Frequently, the objects in the specimen can fall into two general types: normal cells and abnormal cells. The abnormal cells may for example be cells which have indicia of cancer, due to their size, shape and/or color.
It is also common practice to apply one or more stains to the specimen on the slide so that the objects of interest have a contrasting color from background objects or objects of less interest so that they are more readily identified and observed. For example, normal cells are often stained (or, counterstained as is usually said) with a stain such Hematoxylin and appear light blue, while abnormal cells (i.e. positive cells) are stained with a different stain, such as 3-amino 9-ethylcarbazol (AEC) so that the abnormal cells have a different color, e.g. reddish brown. Other color combinations are possible and known in the art.
A quantitative analysis of a limited region of interest in an image is performed in some image processing procedures. For example, image analysis techniques may be applied to a limited region of interest in order to quantify the amount of DNA or a protein present in a region of interest. The algorithms used in such techniques are easier to execute if the unwanted areas in the image, i.e., those area extraneous to the area of interest, are eliminated. Hence, masking techniques are used to isolate the region of interest. In known masking techniques, the region of interest in the image is undisturbed, but image information in the peripheral areas outside the perimeter of the region of interest is deleted, e.g., converted to black. To achieve this, a mask is created that is basically black outside of the region of interest and white (pixel values of 255) in the region of interest. The mask is applied to the image using a logical product operation, e.g., AND, wherein 255 operated by AND with any pixel value yields the pixel value and 0 operated by AND with any pixel value yields 0.
The prior art has had difficulty in easily, reliably and automatically creating a mask for a region of interest that is bounded by closed curve. The hard part is telling what “inside” is, i.e., defining the portion of the image comprising the region of interest that is interior of the closed curve. This is particularly so if the closed curve is essentially any arbitrary closed curve. Given that the region of interest in biological specimens can take virtually any conceivable shape, the algorithms have to be able to create a mask for a region of interest that is bounded by any valid closed curve. An arbitrary closed curve is reducible to a complex polygon in a digital image in which the pixels are arranged in rows and columns. Solution of the problem of whether a particular appoint in the image is inside or within any arbitrary closed curve (complex polygon) is a non-trivial problem. There are heuristics approaches that work some but not all the time, but that is not satisfactory if the image analysis procedures are to work reliably and automatically. Other solutions to the problem are more reliable than heuristics methods, but they are exceedingly complex to implement.
The present invention provides a method of creating a mask for a portion of interest in an image that is very easy to understand conceptually and code in software, very rapid to execute in a computer, and works reliably all the time. As such, it presents a useful contribution to the art. The present invention takes advantage of the insight and discovery of a way to define the area outside of the area of interest and create a mask based on the territory peripheral to the closed curve, rather than attempting to find the interior of the region of interest specified by the closed curve.